Industry is a large consumer of fresh water, and the majority of the water used in industry is used is for cooling. Almost all industry incorporates processes that require dissipation of heat, and cooling water is the most common substance used to dissipate unwanted heat. Power cannot be generated, gasoline cannot be produced, and chemical processes cannot operate without the use of cooling water.
In petroleum refining and petrochemical plants, large amounts of cooling water are needed. By way of example, combustible liquid fuels can be prepared from natural gas. This preparation involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes combustible fuel, among other products. These processes are known as Gas to Liquids (GTL) processes. In GTL processes, large amounts of heat are released in the conversion of natural gas to hydrocarbons; therefore, these plants need large amounts of cooling water.
In a cooling water system, heat exchange is the method by which unwanted heat generated by the process is removed from the system. The cooling water acts as a heat transfer medium in the process. Heat is removed in a cooling tower in two ways. First, it is removed by simple heat exchange between the air and the cooling water. Second, the heat is removed by evaporative heat transfer. After removing heat from the system, the cooling water may be cooled and reused or it may be disposed. Although cooling water may be reused and recirculated for a period of time, eventually evaporation of the cooling water concentrates salts in the cooling water system. The increased salt content increases corrosion problems and the increasing salts may exceed their solubility limits. Therefore, cooling water with increasing salt content eventually requires disposal.
Cooling water requires the use of biocides. Microorganisms or microbes can grow rapidly in cooling water. The warm water and air rich environments provide an ideal environment for microorganisms to multiply. Microorganisms can seriously hamper efficient removal of heat from the system as well as potentially cause structural damage to the system. Therefore, biocides are commonly used to control microbial growth in cooling water systems.
When the cooling water needs to be disposed, biocides present in the cooling water may create disposal problems. An efficient option for disposing of cooling water is directly discharging it into the environment, for example, into a river, lake, ocean, underground aquifer, and the like. However, when the cooling water contains residual biocide, this option may not be available because the biocide may have continuing antimicrobial effects after being released into the environment. Upon direct release into the environment, the biocide may kill or inhibit the growth of indigenous, and potentially desirable, bacteria, molds, fungi, and higher life forms. Therefore, the biocides may contaminate or pollute water supplies or require costly water treatment measures before the cooling water may be disposed.
Another option for disposing cooling water is discharging the water into a biological oxidation facility. However, biocides in the cooling water to be disposed can kill or inactivate the microorganisms used in the biological oxidation facility.
Accordingly, there is a need for appropriate biocides for cooling water and methods of inhibiting the growth and reproduction of microorganisms in cooling water in which the biocide can be deactivated or neutralized before or upon disposal of the cooling water.